Open access

Long term photometric monitoring of comet
103P/Hartley2 with the new robotic TRAPPIST telescope
E. Jehin, J. Manfroid, D. Hutsemékers, M. Gillon, and P. Magain
Institut d’Astrophysique de l’Université de Liège, Belgium ([email protected] / Fax: + [phone])
Abstract
We report on a long term monitoring of comet
103P/Hartley2 with six cometary narrow band filters
using the TRAPPIST 0.60m telescope installed
recently at the ESO La Silla observatory in Chile.
This new robotic telescope is dedicated to exoplanet
and solar system research. The comet was observed
with the cometary filters during 4 months, from Oct.
29 to Feb. 22. Since then the monitoring continues
but only through the BVRI filters and about two
times per week. Those observations allowed us to
make a detailed light curve of the comet after its
perihelion passage and derive production rates of the
4 main species (OH, CN, C2, C3) as well as the dust
production rate (Afρ) over that period. The high
sampling of our monitoring allowed us to find a
periodicity in the gaseous light curves and to deduce
a rotation period of 18.4h early November, slowing
down to about 19h by the end of December.
1. Introduction
TRAPPIST (TRAnsiting Planets and PlanetesImals
Small Telescope) is a new project driven by the
Department of Astrophysics, Geophysics and
Oceanography (AGO) of the University of Liège
(Belgium), in close collaboration with the
Observatory of Geneva (Switzerland). Mostly funded
by the Belgian Fund for Scientific Research (FNRS)
with the participation of the Swiss National Science
Foundation (SNF), TRAPPIST is devoted to the
detection and characterization of planets located
outside our solar system (exoplanets) and to the study
of comets, asteroids and other small solar system
bodies [1]. It consists of a 60cm robotic telescope
that has been installed in 2010 at the ESO La Silla
Observatory in Chile.
A large part of the science time of TRAPPIST is used
for photometric and astrometric survey of comets.
The telescope is equipped with large (5x5cm) high
quality cometary narrow band filters. Those filters
are on loan from the Lowell Observatory (Flagstaff,
USA) and were built by the NASA for the observing
campaign of comet Hale-Bopp [2,3]. Those filters
isolate small spectral regions where cometary species
are mostly emitting (emission bands), as well as
nearby continuum regions (dust reflected solar
spectrum).
Figure 1: The new robotic 0.6m TRAPPIST
telescope installed by the Liège University (Belgium)
in La Silla observatory (ESO, Chile).
For relatively bright comets (V ≤ 13) we measure
two or three times a week the gaseous production
rates and the spatial distribution of several species,
OH, CN, C2 , C3 as well as the ions CO+ and H2O+.
EPSC Abstracts
Vol. 6, EPSC-DPS2011-1489, 2011
EPSC-DPS Joint Meeting 2011
c
Author(s) 2011

The dust continuum and production rate (Afρ) is
estimated from two dust continuum windows in the
blue (the so called filter “BC”) and close to the C2
bands (GC filter). These observations will allow us to
determine the composition of the Southern comets
and the chemical class to which they belong (for
instance C-chain rich or depleted comets as defined
by [3]), possibly revealing the origin of those classes.
Indeed with about 10 expected comets to be observed
each year, this program will provide a good statistical
sample after a few years.
Faint comets (down to mag ~18) are also observed
but only with standard Cousin BVRI filters and the
AFρ value is computed from the R filter.
Figure 2: CN, C3, C2, OH, GC, H2O+ and continuum
light curves of 103P/Hartley2 from Oct. 29 to Feb 22.
The data have been corrected for the heliocentric and
geocentric distances.
2. Observations of 103P/Hartley2
The first cometary target of TRAPPIST was comet
103P/Hartley2. This Jupiter-family comet did a very
close approach to the Earth (0.12 AU) in October
2010 and was the target of a worldwide observing
campaign [4]. On November 4th, the EPOXI mission
(former Deep Impact spacecraft) imaged its nucleus
and provided in-situ IR spectroscopy [5]. Between
Oct. 29 and Feb. 22, we collected on 69 nights a total
of 3967 frames using six different filters. The flux of
five gaseous species (CN, OH, H2O+, C2, C3) and the
dust continuum (via the continuum filter, GC) was
measured in a 26" aperture. Periodic variations of the
gas species are observed, superimposed on the long-
term trend, with CN showing the strongest variations
(0.4-mag amplitude). The dust light curve is nearly
flat. No gas or dust outburst has been detected during
the above period of observation, although CN and
OH show a slow flux increase with respect to other
species during several days in early November.
3. Rotation period
A period search based on the first two weeks of data
gave a smooth phase diagram assuming a period of
18.4 +/- 0.3 hours. These observations are in good
agreement with the period reported from Arecibo
radar observations of the nucleus (18.1 +/- 0.3 hours
on Oct. 24-27 [6]). Including two more weeks of
data, it became clear that the period was increasing.
A change from about 18.2 to 19 hours was derived
that is most probably related to a change of the
rotation period of the nucleus of the comet of about 2
hours in 100 days [7]. Such a rapid slowing down is
in agreement with rotation values reported before
perihelion (16.6 +/- 0.5 hr on Aug. 13-17 [8]).
Acknowledgements
TRAPPIST is a project funded by the Belgian Fund
for Scientific Research (FRS-FNRS) with the
participation of the Swiss National Science
Fundation (SNF). J.M is Research Director FNRS,
E.J and M.G. are Research Associate FNRS, D.H is
Senior Research Associate FNRS.
References
[1] http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/
[2] Farnham, T. L.; Schleicher, D. G.; A'Hearn, M. F. :
Icarus 147, 180, 2000.
[3] A’Hearn, M. F. et al. : Icarus 188, 223, 1995
[4] Meech, K. et al.: ApJ 734, L1, 2011.
[5] A’Hearn, M. et al.: Science, in press, 2011.
[6] Harmon, J.K. et al.: IAUC 9179, 2010.
[7] Jehin, E. et al.: CBET 2589, 2010.
[8] Knight, M. et al.: IAUC 9163, 2010.